a. Field of the Invention
This invention relates to methods for stretching tows of filaments.
B. Description of the Prior Art
A number of man-made filaments of various known types are made by forcing a spinning solution through a spinnerette to form a tow of filaments. In almost all of the various types of man-made filaments it is necessary at one point or another to stretch the filaments to obtain desired properties. In order to obtain best results the tow is usually heated in some manner and is stretched while hot. The heating of the tow is usually accomplished by passing the tow over heated rolls or through a steam chamber or by other known methods such as the use of sprays or cascades.
Apparatus other than cascades and sprays may be used for stretching a tow. U.S. Pat. No. 3,267,704 issued to H. G. Mueller and U.S. Pat. No. 3,353,383 issued to E. A. Taylor, Jr., for example, show apparatus for washing a tow wherein the washing liquid is continually passed back and forth through the tow. Such apparatus can be used for stretching a tow if the apparatus is sufficient in length and the liquid is sufficiently hot. In fact, a simple bath can be used if it is long enough. The requirement which must be met is that the tow must be wet and at a sufficiently high temperature.
The disadvantage of most of the known methods of heating and stretching tow is that the heating operation is inefficient and slow, especially when sprays or baths are used. Either the tow must have a long dwell time in the heating zone or excessive temperatures must be used to raise the temperature of the filaments to a point where they can be stretched. Further, it is very difficult in conventional stretching processes to heat the inner filaments of the tow to the same temperature as the outer filaments, since the outer layers of filaments shield the inner filaments from the heated liquid.